Microstructure Simulation And Mechanical Behavior Study Of Heterogeneous Anisotropic Microscale Cu/Sn58Bi/Cu Solder Joints In Electronic Packaging

Electronics are becoming smaller and more versatile,and the size of solder joints has decreased to tens of microns,inducing obvious inhomogeneity and anisotropy among the phases in the solder matrix microstructure of Sn58Bi solder joints.In this study,the influence of microstructure inhomogeneity and anisotropy on the mechanical properties of line-type Cu/Sn58Bi/Cu microscale solder joints was studied.This study mainly includes the following three parts:（1）The mechanical properties of crystalline Sn and Bi were studied by first-principles methods based on density functional theory.The elastic constants of single crystal Sn and Bi and the modulus of polycrystalline Sn and Bi were calculated.In addition,the elastic modulus of single crystal Sn was measured by nano indentation and EBSD test.The test results are very close to the elastic modulus calculated from the elastic constant,which verifies the accuracy of first-principle calculation.（2）The line-type Cu/Sn58Bi/Cu microscale solder joints with heights of 200μm and300μm were prepared by reflow soldering,and the microstructure of the solder alloy was observed by scanning electron microscope（SEM）.The results show that the microstructure of Sn58Bi alloy is a typical eutectic layered structure,including Sn phase and Bi phase,respectively,which appear together in an alternately layered structure.At the same time,a quasi-static tensile test of the solder joints was carried out using a dynamic mechanical analyzer.The results displayed that the mechanical properties of solder joints with a height of 200μm are slightly better than those of solder joints with a height of 300μm.（3）Based on the obtained mechanical performances,elastic constants,and eutectic microstructure characteristics observed by SEM,a three-dimensional tensile finite element model of line-type Cu/Sn58Bi/Cu microscale solder joints,including their microstructure,was established.The results demonstrate that microstructure inhomogeneity increases the maximum value of first principal stress（σ₁）and Von Mises stress(σ_eq),leading to stress concentration.When the Sn58Bi solder transfers from the elastic deformation stage to the plastic deformation stage,the highσ_eq zone of the matrix gradually shifts from the Sn phase to the Bi phase and is located at the boundary of two phases.The highσ₁ zone has been distributed in the Sn phase and is located at the interface with copper substrate.In addition,a study of the anisotropy reveals that the elastic anisotropic mechanical properties of Sn58Bi solder are mainly affected by the anisotropic effect of Sn.Theσ₁ andσ_eqconcentration is the lowest when the crystal orientation isπ/2.